Geology, climate, and water circulation of the Galveston Bay System

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The geology of the Galveston Bay System reflects its location in one of the world's largest depositional basins, the northwest Gulf Coast Basin, as well as changes in the rates and balance among sea level, sediment influx, and basin subsidence. Sedimentary deposits of two ages dominate the surficial geology surrounding the bays. Deposits of the most recent interglacial period of the Pleistocene epoch include 1) river sands and floodbasin muds of a deltaic plain, and 2) sands of a barrier island system. Modern (Holocene) sediments that entrench and overlie the older strata are fine sand and mud in rivers and bayhead deltas; mud in the bays; oyster reefs in the bays; and sand composing the youngest barrier islands. Galveston Bay is extremely shallow (10-12 ft deep) compared with its large areal extent of 600 mi super(2) . Sediment samples, collected a mile apart, are mud in most of the bays; samples coarsen shoreward where sand and reworked shell (gravel) dominate. Geochemical analyses of sediment samples indicate that abnormally high concentrations of barium, boron, chromium, copper, lead, nickel, and zinc are products of anthropogenic activities and pollutants. The Galveston Bay System has a subhumid, subtropical climate; mean summer high temperatures are in the upper 80s ( degrees F), and mean winter low temperatures are in the middle 40s ( degrees F). Mean annual rainfall and surface-water evaporation are similar to 50 in. Summer winds are dominantly moderate and southerly; winter brings frequent aperiodic strong north winds. Droughts and hurricanes are frequent. Bay circulation is controlled by balances among freshwater influx, tides, and storm winds. The Trinity and San Jacinto River basins provide >88% of the freshwater inflow to the bays. Bay tides are diurnal in a 14-day cycle, and maximum tidal range is similar to 2 ft. Hurricane landings may raise the bay level to 15 ft., whereas strong north winds may locally lower bay level similar to 2 ft. Principal geologic processes currently altering the Galveston Bay System include 1) a relative sea level rise ( similar to 2 ft in this century) and subsidence (nearly 10 ft at Johnson Space Center) in response to withdrawal of subsurface water, oil, and gas; 2) active faulting; and 3) coastal erosion and deposition. Between 1850 and 1982, bay shorelines eroded at an average rate of 2.2 ft/yr; before 1930, the erosion rate was 1.8 ft/yr, whereas the post-1930 rate was 2.4 ft/yr. Human activities commonly overprint normal natural processes and effect a loss of natural resources. Models of circulation, salinity, and nutrients developed by the Texas Water Development Board indicate potential management problems. Further documentation and regular, selective process monitoring are needed for future holistic management of the Galveston Bay System to be successful